Not applicable
Not applicable.
Not Applicable
1. Field of the Invention
This invention relates to a particle-optical apparatus, and more particularly to a particle-optical apparatus for microstructures for semiconductor lithography.
For the particle-optical production of microstructures, for example, for semiconductor lithography, it is known to construct a mask which is to be imaged on a reduced scale, either as a self-supporting mask or as a thin membrane with strongly scattering thicker structures. With self-supporting masks, however, no hollow structures are possible, i.e., structures which have no connection to the mask edge. Since however the projection of such hollow structures is often required in semiconductor lithography, the range of application of self-supporting masks is greatly limited. Furthermore, particle absorption in the mask structure leads to differential heating of the mask, easily resulting in a mask deformation.
2. Discussion of Relevant Art
These disadvantages do not arise with transparent masks, in which a strongly scattering structure is applied to a weakly scattering membrane. However, there is instead the disadvantage of a very low contrast, since contrast generation, as is known from U.S. Pat. No. 5,079,112, for example, takes place by means of an aperture diaphragm which is located downstream of the mask, and which either permits the transmission only of particles which emerge from the mask at a large scattering angle (this substantially corresponds to dark field contrast), or permits the transmission only of particles which emerge at a small scattering angle from the mask (this substantially corresponds to a bright field contrast). Since, however, both strongly scattering and weakly scattering regions always produce large ranges of scattering angles, which differ only in their statistical distribution, there results the weak contrast which has already been mentioned.
The present invention therefore has as its object to provide a particle-optical apparatus and a particle-optical process, with which microstructures applied to a thin membrane can be imaged with high contrast.
This object is attained with a particle-optical apparatus with a mask holder to receive a mask in a mask plane and an imaging energy filter following the mask holder, wherein the mask plane is imaged on a reduced scale in an image plane by means of the energy filter.
This object also is attained by a process for the particle-optical production of microstructures, wherein a mask with inelastically scattering microstructures is imaged on a reduced scale on a wafer by means of an energy filter, and wherein particles of a predetermined energy loss are selected with the energy filter.
The particle-optical apparatus according to the invention has an imaging energy filter arranged after a mask which carries the microstructure. Inelastically scattered particles of a predetermined energy window are filtered out by means of the energy filter. The imaging of the mask on a reduced scale in the image plane or projection plane then takes place by means of an imaging system which is arranged downstream of the energy filter.
The invention is accordingly based on the idea of using for the generation of contrast a separation of the particles according to their energy or according to their energy differences, after interaction with the mask which is to be imaged. Since the energy loss of the particles is strongly element-specific, a high imaging contrast can be produced by the use of different materials for the membrane on the one hand and the microstructure on the other hand. There is then no problem if both the membrane and the microstructure scatter inelastically, as long as the energy loss spectra of the materials used for the membrane and for the microstructure are sufficiently different, so that a separation of the inelastically scattered particles from the inelastically scattered particles, according to energy, is possible at the output of the energy filter. However, it is preferable, in connection with the apparatus according to the invention and the process according to the invention, if the mask which carries the microstructure consists of a support foil which scatters the particles elastically, with a microstructure which scatters inelastically. In such a case, the inelastically scattered particles are preferably filtered out at the output of the energy filter, so that only the elastically scattered particles contribute to the imaging of the mask, since these have the smallest spectral distribution.
The imaging of the mask plane in the image plane is to take place such that the image of the mask on the image plane is at most half as large as the mask (scale or reduction smaller than 0.5). The energy filter should preferably be constituted as a so-called imaging energy filter, which images a first plane on the input side (the input image plane) achromatically into a first plane on the output side (output image plane), and a second plane on the image side dispersively into a second plane on the output side (dispersive plane). The mask is then either to be arranged in the first plane on the input side, or to be imaged by an imaging stage, which follows the mask, into this first plane on the input side of the filter.
A preparation holder, which receives the wafer to be structured, is preferably arranged in or behind the image plane of the apparatus. This preparation holder is to be movable by motor in two mutually perpendicular directions, which are perpendicular to the optical axis of the particle-optical apparatus. Likewise, a preparation holder which receives the mask and is movable by motor in two mutually perpendicular directions, which are again perpendicular to the optical axis of the particle-optical apparatus, is to be arranged in the mask plane. By means of corresponding coupling of the movement of the preparation holder in the mask plane with the movement of the preparation holder in the image plane, different regions of the mask can be successively imaged, energy filtered, on different regions of the wafer to be arranged in the image plane, so that mask fields which are larger than the respectively transmitted image field can be imaged by successive projections.
A condenser system can be arranged between the particle source and the mask plane. Such a condenser system makes it possible to provide a collimated beam path in the mask plane.
Furthermore, there can be provided a first deflecting system between the particle source and the mask plane, a second deflecting system between the mask plane and the energy filter, and a third deflecting system behind the energy filter. Each of these three deflecting systems can be constructed in a known manner as a double deflection system. The first and second deflecting systems are then preferably excited such that a particle beam entering the first deflecting system parallel to the optical axis is deflected to an out-of-axis region on the mask plane, and behind the mask plane is deflected back again in the direction toward the optical axis. An out-of-axis deflection then again takes place behind the energy filter by means of the third deflecting system. With the described combination of deflecting systems, different regions of the mask to be arranged in the mask plane can be imaged on different regions of the wafer which is to be received in the image plane behind the energy filter. In this manner, successively larger mask regions can be imaged on the wafer electron-optically, i.e., without mechanical movement of the mask and wafer relative to each other. However, it is particularly advantageous in this connection if the particle-optical deflection and the mechanical movement are combined together, such that by means of suitable machine control, respectively two or more particle-opticalally produced image displacements take place respectively between two mechanical movements of the mask holder in the mask plane and of the preparation holder in the imaging plane.
The particles used for mask imaging in connection with the invention can be electrons or ions, the use of electrons being preferred since the components required for the invention are basically known from transmission electron microscopes.